1. Field of the Invention
This invention relates to novel electron deficient olefins, such as certain 2-cyanoacrylates or methylidene malonates, prepared using an imine or an iminium salt.
2. Brief Description of Related Technology
Cyanoacrylate adhesives are known for their fast adhesion and ability to bond a wide variety of substrates. They are marketed as “super glue” type adhesives. They are useful as an all-purpose adhesive since they are a single component adhesive, very economical as only a small amount will do, and generally do not require any equipment to effectuate curing.
Traditionally, cyanoacrylate monomers have been produced by way of a Knoevenagel condensation reaction between a formaldehyde precursor, such as paraformaldehyde, and an alkyl cyanoacetate with a basic catalyst. During the reaction, cyanoacrylate monomer forms and polymerises in situ to a prepolymer. The prepolymer is subsequently thermally cracked or depolymerised, yielding cyanoacrylate monomer. This approach has remained essentially the same over time, though various improvements and variants have been introduced. See e.g. U.S. Pat. Nos. 6,245,933, 5,624,699, 4,364,876, 2,721,858, 2,763,677 and 2,756,251.
In U.S. Pat. No. 3,142,698, the synthesis of difunctional cyanoacrylates using a Knoevenagel condensation reaction is described. However, the ability to thermally depolymerise the resulting, now crosslinked, prepolymer in a reliable and reproducible manner to produce pure difunctional monomers in high yields is questionable [see J. Buck, J. Polym. Sci., Polym. Chem. Ed., 16, 2475-2507 (1978), and U.S. Pat. Nos. 3,975,422, 3,903,055, 4,003,942, 4,012,402, and 4,013,703].
A variety of other processes for producing cyanoacrylate monomers are known, some of which are described below. For instance, U.S. Pat. No. 5,703,267 defines a process for producing a 2-cyanoacrylic acid which comprises subjecting a 2-cyanoacrylate and an organic acid to a transesterification reaction.
U.S. Pat. No. 5,455,369 defines an improvement in a process for preparing methyl cyanoacrylate, in which methyl cyanoacetate is reacted with formaldehyde to form a polymer that is then depolymerized to the monomeric product, and in which the purity of yield is 96% or better. The improvement of the '369 patent is reported to be conducting the process in a poly(ethylene glycol) diacetate, dipropionate, or dibutyrate, having a number average molecular weight of 200-400, as the solvent.
U.S. Pat. No. 6,096,848 defines a process for the production of a biscyanoacrylate, which comprises the steps of esterifying a 2-cyanoacrylic acid or transesterifying an alkyl ester thereof to obtain a reaction mixture; and fractionally crystallizing the reaction mixture to obtain the biscyanoacrylate.
U.S. Pat. No. 4,587,059 defines a process for the preparation of monomeric 2-cyanoacrylates comprising the steps of (a) reacting (i) a 2,4-dicyanoglutarate with (ii) formaldehyde, cyclic or linear polymers of formaldehyde, or a mixture thereof, in the presence of between about 0.5 and about 5 mols of water per mol of 2,4-dicyanoglutarate, at an acid pH of about 3 to slightly less than 7, and at a temperature of about 70 to about 140, to form an oligomeric intermediate product, and (b) removing water that is present from step (a) and thermolyzing the oligomeric intermediate product for a period of time sufficient to effect its conversion to monomeric 2-cyanoacrylates.
Commercial production of cyanoacrylate monomers ordinarily relies on the depolymerisation of a prepolymer formed under Knoevenagel condensation reaction conditions, as noted above. Still today the Knoevenagel condensation reaction is believed to remain the most efficient and prevalent commercial method for producing high yields of monofunctional cyanoacrylates. Nevertheless, it would be desirable to not have to resort to thermally induced depolymerisation of a prepolymer produced by the Knoevenagel condensation reaction. This prospect may also enable facile access to highly useful difunctional monomers, such as so-called biscyanaocrylates or hybrid materials of cyanoacrylate and other polymerisable or reactive functionality.
For instance, cyanoacrylate esters bearing moisture, base, acid, thermally sensitive or otherwise reactive moieties, may not be conveniently produced and isolated under Knoevenagel reaction conditions.
While methods describing the preparation of cyanoacrylates with reactive functionality in the ester side chain (such as biscyanoacrylates) are known (see e.g. Buck and U.S. Pat. Nos. 3,975,422, 3,903,055, 4,003,942, 4,012,402, and 4,013,703), the cyanoacrylates with reactive functionality in the ester side chain are prepared in a multi-step process involving protective group strategies and functional group transformations to arrive at adducts which must subsequently be deprotected to yield cyanoacrylates with additional functionality. The same approach has been described to arrive at a cyanoacrylate-capped polyisobutylene by Kennedy et al., J. Macromol Sci. Chem., A28, 209 (1991).
A transesterification approach to achieve cyanoacrylates with reactive functions in the ester side chain has also been described in U.S. Pat. No. 6,096,848, in which cyanoacrylate esters, previously made by Knoevenagel reaction, are hydrolysed in strong acid conditions in the presence of a difunctional alcohol to yield biscyanoacrylates. The method described in the '848 patent requires long reaction times, copious volumes of solvent and solvent switching methods to isolate the bifunctional cyanoacrylates free from acid stablisers in modest to low yields [see also Khrustalev et al., Russian Chem. Bull., 45, 9, 2172 (1996)].
An alternative approach to the preparation of cyanoacrylates with reactive functions in the ester side chain uses cyanoacrylic acid or its acid chloride (cyanoacryloyl chloride). See e.g. International Patent Publication Nos. WO 94/15590A1, WO 94/115907A1, and WO 95/32183A1, and U.S. Pat. No. 5,703,267.
The use of cyanoacrylic acid and cyanoacryloyl chloride to arrive at cyanoacrylates has also been described in Y. Gololobov and I. Chernoglazova, Russian Chem. Bull., 42, 5, 961 (1993) and Y. Gololobov and M. Galkina, Russian Chem. Bull., 44, 4, 760 (1995). These methods require flash vacuum pyrolysis techniques conducted in quartz tubes at high temperatures (approximately 600° C.) and exposure of highly reactive, polymerisable intermediate materials to chemical reactions with highly acidic and moisture sensitive reagents.
With regard to the preparation of other types of electron deficient olefins with reactive functionality, U.S. Pat. No. 5,142,098 describes a copper catalysed reaction of malonates and formaldehyde to form methylidenemalonate monomers that are trapped in situ by a “diene” anthracene in a Diels-Alder reaction. The '098 patent describes a diester adduct of anthracene, that is a precursor for a methylidenemalonate monomer with one ethyl ester and one glycidyl ester. The '098 patent indicates that reaction—a retro Diels-Alder thermolysis step—was not successful for the preparation of the particular methylidene malonate bearing the glycidyl functionality in the ester side chain. The retro Diels-Alder reaction has been reported as useful in the syntheses of other methylidene malonates (see e.g. J-L. De Keyser et al., J. Org. Chem., 53, 4859 (1988)).
Accordingly, it will be appreciated that the preparation of electron deficient olefins, such as 2-cyanoacrylates or methylidene malonates, with a reactive functional group in the ester, or even with large or bulky groups in the ester side chain, is not a trivial matter.
As a result and because of the limitations of the hitherto known various processes for cyanoacrylate synthesis and the sensitivity of the novel electron deficient olefins, such novel electron deficient olefins have not been described to date. Until now.